This invention relates to laser catheters and optical fiber systems for generating and transmitting energy to a surgical site in a living body for the purposes of tissue removal or repair.
While lasers have been used for many years for industrial purposes such as drilling and cutting materials, it is only recently that surgeons have begin to use lasers for surgical operations on living tissue. To this end, laser energy has been used to repair retinal tissue and to cauterize blood vessels in the stomach and colon.
In many surgical situations, it is desirable to transmit laser energy down an optical fiber to the surgical location. if this can be done, the optical fiber can be included in a catheter which can be inserted into the body through a small opening, thus reducing the surgical trauma associated with the operation. In addition, the catheter can often be maneuvered to surgical sites which are so restricted that conventional scalpel surgery is difficult, if not impossible. For example, laser energy can be used to remove atherosclerotic plaque from the walls of the vasculature and to repair defects in small-diameter artery walls.
A problem has been encountered with laser surgery in that prior art lasers which have been used for industrial purposes often have characteristics which are not well suited to percutaneous laser surgery. For example, a laser which in conventionally used for scientific purposes is an excimer laser which Is a gas laser that operates with a gas mixture such as Argon-Fluorine, Krypton-Fluorine or Xenon-Fluorine. Another common industrial laser is the carbon dioxide or CO2 laser.
Both the excimer laser and the CO2 laser have been used for surgical purposes with varying results. One problem with excimer lasers is that they produce output energy having a wavelength in the range 0.2-0.5 micrometers. Blood hemoglobin and proteins have a relatively high absorption of energy in this wavelength range and, thus, the output beam of an excimer laser has a very short absorption length in these materials (the absorption length is the distance in the materials over which the laser beam can travel before most of the energy is absorbed). Consequently, the surgical site in which these lasers are to be used must be cleared of blood prior to the operation, otherwise most of the laser energy will be absorbed by intervening blood before it reaches the surgical area. While the removal of blood is possible if surgery is performed on an open area it is often difficult if surgery is to be performed via a catheter located in an artery or vein.
An additional problem with excimer lasers is that the output energy pulse developed by the laser is very short, typically about ten nanoseconds. In order to develop reasonable average power, pulses with extremely high peak power must be used. When an attempt is made to channel such a high peak power output into an optical fiber, the high peak power destroys the fiber. Thus, excimer lasers have a practical power limit which is relatively low. Consequently, when these lasers are used for biological tissue removal, the operation is slow and time consuming.
The CO2 laser has other drawbacks. This laser generates output energy with a wavelength on the order of 10 micrometers. At this wavelength, the absorption of blood hemoglobin is negligible but the absorption by water and tissue is relatively high. Scattering at this wavelength is also very low. Although the CO2 laser possesses favorable characteristics for surgical applications in that it has low scattering and high absorption in tissue, it suffers from the same drawback as excimer lasers in that the absorption length is relatively short due to the high absorption of the laser energy in water. Thus, the surgical area must be cleared of blood prior to the operation.
Unfortunately, the CO2 laser also suffers from a serious additional problem. Due to the long wavelength, the output energy from the carbon dioxide laser cannot be presently transmitted down any optical fibers which are suitable for use in percutaneous surgery (present fibers which can transmit energy from a CO2 laser are either composed of toxic materials, are soluble in water or are not readily bendable, or possess a combination of the previous problems) and, thus, the laser is only suitable for operations in which the laser energy can be either applied directly to the surgical area or applied by means of an optical system comprised of prisms or mirrors.
Accordingly, it is an object of the present invention to provide a laser catheter system which uses laser energy of a wavelength that is strongly absorbed in water, in bodily tissues and atherosclerotic plaque.
It is another object of the present invention to provide a laser catheter system which is capable of providing laser energy that can be transmitted through existing silica-based optical fibers.
It is a further object of the present invention to provide a laser catheter system in which optical scattering is minimized and which has a medium-length absorption length to confine the energy to the area of interest.
It is yet another object of the present invention to provide an optical catheter system with a laser that can be operated on either a pulsed mode or a continuous wave mode.
It is still another object of the present invention to provide a laser catheter system which can be used for biological material removal and biological material repair.
The foregoing objects are achieved and the foregoing problems are solved in one illustrative embodiment of the invention in which a laser catheter system employs a laser source operating in the wavelength region of 1.4-2.2 micrometers. Illustrative laser sources operating this region are Holmium-doped YAG, Holmium-doped YLF, Erbium-doped YAG, Erbium-doped YLF and Thulium-doped YAG lasers.
In the inventive laser system, the above-noted lasers are used with a specially-treated silica fiber that has been purified to reduce the concentration of hydroxyl (OHxe2x80x94) ions.
For biological tissue removal, the laser source may be operated in a pulsed mode with a relatively long pulse of approximately 0.2-5 milliseconds at an energy level of approximately 1-2 joules per pulse. With this time duration and energy level, the peak power of the laser pulse is approximately 1 kilowatt. This amount of power can easily be tolerated by the silica fiber, but is sufficient for rapid material removal. With a repetition rate in the range of 1-10 hertz, the average power delivered to a surgical site by such a laser will be under 10 watts.
Alternatively, for biological tissue repair, the laser source can be operated in a low power continuous wave mode to repair, by coagulation, of tissue by a process similar to xe2x80x9cspot weldingxe2x80x9d.